Laser processing can be conducted on numerous different workpieces using various lasers effecting a variety of processes. The specific types of laser processing of primary interest are laser processing of a single or multilayer workpiece to effect through-hole or blind via formation.
U.S. Pat. Nos. 5,593,606 and 5,841,099 to Owen et al. describe methods of operating an ultraviolet (UV) laser system to generate laser output pulses characterized by pulse parameters set to form in a multilayer device through-hole or blind vias in two or more layers of different material types. The laser system includes a nonexcimer laser that emits, at pulse repetition rates of greater than 200 Hz, laser output pulses having temporal pulse widths of less than 100 ns, spot areas having diameters of less than 100 μm, and average intensities or irradiance of greater than 100 mW over the spot area. A typical nonexcimer UV laser used includes a diode-pumped, solid-state (DPSS) laser.
U.S. Pat. No. 6,784,399 to Dunsky et al. describes a method of operating a pulsed CO2 laser system to generate laser output pulses that form blind vias in a dielectric layer of a multilayer device. The laser system emits, at pulse repetition rates of greater than 200 Hz, laser output pulses having temporal pulse widths of less than 200 ns and spot areas having diameters of between 50 μm and 300 μm. The above-identified patents to Owen et al. and to Dunsky et al. are assigned to the assignee of this patent application.
Laser ablation of a target material, particularly when a UV DPSS laser is used, relies upon directing to the target material a laser output having a fluence or energy density that is greater than the ablation threshold of the target material. A UV laser emits laser output that can be focused to have a spot size of between about 10 μm and about 30 μm at the 1/e2 diameter. In certain instances, this spot size is smaller than the desired via diameter, such as when the desired via diameter is between about 50 μm and 300 μm. The diameter of the spot size can be enlarged to have the same diameter as the desired diameter of the via, but this enlargement reduces the energy density of the laser output such that it is less than the ablation threshold of the target material and cannot effect target material removal. Consequently, the 10 μm to 30 μm focused spot size is used and the focused laser output is typically moved in a spiral, concentric circular, or “trepan” pattern to form a via having the desired diameter. Spiraling, trepanning, and concentric circle processing are types of so-called non-punching via formation processes. For via diameters of about 50 μm or smaller, direct punching delivers a higher via formation throughput.
In contrast, the output of a pulsed CO2 laser is typically larger than 50 μm and capable of maintaining an energy density sufficient to effect formation of vias having diameters of 50 μm or larger on conventional target materials. Consequently, a punching process is typically employed when using a CO2 laser to effect via formation. However, a via having a spot area diameter of less than 45 μm is difficult to achieve with a CO2 laser.
The high degree of reflectivity of copper at the CO2 wavelength makes very difficult the use of a CO2 laser in forming a through-hole via in a copper sheet having a thickness of greater than about 5 microns. Thus, CO2 lasers are typically used to form through-hole vias only in copper sheets having thicknesses that are between about 3 microns and about 5 microns, or that have been surface treated to enhance the absorption of the CO2 laser energy.
The most common materials used in making multilayer structures for printed circuit board (PCB) and electronic packaging devices in which vias are formed typically include metals (e.g., copper) and dielectric materials (e.g., polymer polyimide, resin, or FR-4). Laser energy at UV wavelengths exhibits good coupling efficiency with metals and dielectric materials, so the UV laser can readily effect via formation on copper sheets and dielectric materials. Also, UV laser processing of polymer materials is widely considered to be a combined photo-chemical and photo-thermal process, in which the UV laser output partly ablates the polymer material by disassociating its molecular bonds through a photon-excited chemical reaction, thereby producing superior process quality as compared to the photo-thermal process that occurs when the dielectric materials are exposed to longer laser wavelengths.
CO2 laser processing of dielectric and metal materials and UV laser processing of metals are primarily photo-thermal processes, in which the dielectric material or metal material absorbs the laser energy, causing the material to increase in temperature; decompose, soften, or become molten; and eventually ablate, vaporize, or blow away. Ablation rate and via formation throughput, are, for a given type of material, functions of laser energy density (laser energy (J) divided by spot size (cm2)), power density (laser energy (J) divided by spot size (cm2) divided by pulse width (seconds)), laser wavelength, and pulse repetition rate. When punching micro-vias (usually less than 150 μm), to get the best quality, typically a laser beam needs to be converted from a Gaussian beam profile into a “top-hat” or flattened beam profile.
Thus, laser processing throughput, such as, for example, via formation on a PCB or other electronic packaging devices, or hole drilling on metals or other materials, is limited by the laser power intensity available and pulse repetition rate, as well as the speed at which the beam positioner can move the laser output in a spiral, concentric circle, or trepan pattern and between via positions. An example of a UV DPSS laser is a Model Q302 (355 nm) sold by JDSU (JDS Uniphase Corporation), San Jose, Calif. This laser is used in a Model 5330 laser system or other systems in its series manufactured by Electro-Scientific Industries, Inc., Portland, Oreg., the assignee of the present patent application. The laser is capable of delivering 8 W of UV power at a pulse repetition rate of 30 kHz. The typical via formation throughput of this laser and system is about 600 vias each second on bare resin. An example of a pulsed CO2 laser is a Model Q3000 (9.3 μm) sold by Coherent-DEOS, Bloomfield, Conn. This laser is used in a Model 5385 laser system or other systems in its series manufactured by Electro-Scientific Industries, Inc. The laser is capable of delivering 18 W of laser power at a pulse repetition rate of 60 kHz. The typical via formation throughput of this laser and system is about 900 vias each second on bare resin and 200-300 vias each second on FR-4.
Increased via formation throughput could be accomplished by increasing the laser energy per pulse and the pulse repetition rate. However, for the UV DPSS laser and the pulsed CO2 laser, there are practical problems stemming from the amounts by which the laser energy per pulse and the pulse repetition rate can be increased. Moreover, as laser energy per pulse increases, the risk of damage to the optical components inside and outside the laser resonator increases. Repairing damage to these optical components is especially time-consuming and expensive. Additionally, lasers capable of operating at a high laser energy per pulse or a high pulse repetition rate are often prohibitively expensive.
Fiber lasers are more recently being used to provide processing laser outputs because they provide high energy density and beam quality, along with integrated methods of amplification that aid in focusing the energy onto a target material to execute via drilling. A basic fiber laser may include a single mode core made of a laser material, e.g., doped with a laser ion such as neodymium, erbium, terbium or praseodymium, to provide an active galin medium. The fiber laser may further include a concentrically surrounding multi-mode fiber core and clad to define a pump cavity for the single mode core. (In the alternative, a separate fiber may run parallel to the single mode core to provide the pump source.) The indices of refraction of these three layers (single mode core, multi-mode core, and clad) are chosen so that pumping radiation delivered into one end of the fiber will be totally internally reflected at the interface between core and clad and propagate along the fiber.
The pumping radiation passes many times through the core of the laser material to provide effective coupling of the pumping radiation to the laser gain medium. However, the total reflection of the pumping radiation at the interface between the single mode and the multi-mode cores is such that the laser radiation is trapped within and propagates along the single mode core, thus providing a high-energy pumped, high-quality laser beam. The fiber may be placed between reflectors, such as mirrors, to define a resonant cavity to produce a laser beam of a particular resonant wavelength. As with the other lasers discussed herein, fiber lasers may incorporate optical train elements to process a fundamental wavelength of laser light into various harmonic wavelengths, and/or to adjust other parameters, such as pulse width and energy density.